The present invention relates generally to batteries, and more particularly relates to a ventilation system and air manager system for a metal-air battery.
Metal-air battery cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. Metal-air battery cells are often arranged in multiple cell battery packs within a common housing to provide a sufficient amount of power output. The result is a relatively light-weight battery.
To operate a metal-air battery cell, it is necessary therefore to provide a supply of oxygen to the air cathodes of the cells. Some prior systems sweep a continuous flow of new ambient air across the air cathodes at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in U.S. Pat. No. 4,913,983 to Cheiky. Cheiky uses a fan within the battery housing to supply a flow of ambient air to a pack of metal-air battery cells. When the battery is turned on, an air inlet and an air outlet are opened and the fan is activated to create the flow of air into, through, and out of the housing. U.S. Pat. Nos. 5,569,551 and 5,641,588 are incorporated herein by reference.
One problem with a metal-air battery is that the ambient humidity level can cause the battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 45 percent. If the ambient humidity is greater than the equilibrium humidity within the battery housing, the battery will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the battery to burst. If the ambient humidity is less than the equilibrium humidity within the battery housing, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. The art, therefore, has recognized that an ambient air humidity level differing from the humidity level within the battery housing will create a net transfer of water into or out of the battery. These problems are particularly of concern when the battery is not in use, because the humidity tends to either seep into or out of the battery housing over an extended period of time.
Another problem associated with metal-air batteries is the transfer of carbon dioxide from ambient air into the battery cell. Carbon dioxide tends to neutralize the electrolyte, such as potassium hydroxide. In the past, carbon dioxide absorbing layers have been placed against the exterior cathode surface to trap carbon dioxide. An example of such a system is shown in U.S. Pat. No. 4,054,725.
Maintaining a battery cell with proper levels of humidity and excluding carbon dioxide has generally required a sealed battery housing. As discussed above, however, prior art systems such as that disclosed by Cheiky, have used a fan of some sort to force ambient air through the battery housing during use. Large openings are provided to permit the in-flow and out-flow of air. These openings are generally sealed during non-use by a mechanical air door. If the air door is not present or not shut during non-use, large amounts of ambient air would seep into the housing. This flow of air would cause the humidity and carbon dioxide problems within the housing as discussed above. The oxygen in the ambient air also would cause the cell to discharge, thereby leading to xe2x80x9cleakagexe2x80x9d current and a reduction in cell efficiency and lifetime.
Even with the use of air doors, however, a certain amount of oxygen and contaminates tend to seep into the cell during non-use. Some leakage current is therefore inevitable. Although the air doors limit this leakage current and the other problems discussed above, the use of the air doors increases the complexity of the battery housing itself and increases the cost and time of manufacture of the overall battery.
Air doors have not been needed in some applications of metal-air cells, such as shown in include U.S. Pat. 4,118,544 to Przybyla. Przybyla describes a primary metal-air button cell used with watches and hearing aids. Such cells operate during a single, continuous discharge at very low current levels. In essence, Przybyla relies upon the use of continuous xe2x80x9cleakage currentxe2x80x9d to power devices with very low current demands.
Metal-air cells typically are designed to have a relatively large air electrode surface, so that as large a power output as possible can be obtained from a cell of a given volume and weight. Once air is ventilated into a metal-air battery housing, a goal has been to distribute the oxygen-bearing air uniformly and efficiently to all air electrode surfaces. Recirculation air managers including fans within the battery housing have been developed to distribute air within the housing while keeping the volume of make-up air as low as practicable. However, in multiple cell systems, air distribution paths typically have extended from a fan, positioned along a periphery of the housing adjacent to an air door, for a lengthy distance over all of the air electrode surfaces. An example is shown in U.S. Pat. No. 5,387,477. Oxygen is depleted from the air stream so that oxygen concentration at the end of the distribution path often has fallen below a level desired for optimal power production from all the cells. Known systems that solve this problem by blowing outside air over the cells and exhausting it immediately without recirculation are subject to the flooding or drying out problems described above.
Thus, there has been a need for a practical air manager system for a metal-air battery without mechanical air doors or other mechanical sealing methods to prevent diffusion therethrough when the battery is not in use. The system should maintain a stable water vapor equilibrium across the air cathode of a metal-air cell while convectively providing new oxygen for operation of the cell at desired power levels in a simplified battery housing. There also has been a need for an air distribution system within a metal-air battery housing that minimizes the length of the air distribution path to the air electrode surfaces and minimizes the variation of the concentration of oxygen in the distributed air for all cells.
The present invention provides a ventilation system for a metal-air battery having a housing for enclosing at least one metal-air cell. The housing has at least one air inlet opening and at least one air outlet opening. A fan is positioned to force air into the air inlet opening and out of the air outlet opening when the fan is turned on. The openings are sized with a length in the direction through the thickness of the housing being greater than a width in the direction perpendicular to the thickness of the housing. The openings are unobstructed and are sized to eliminate substantially the air flow into the air inlet opening and out of the air outlet opening when the fan is turned off.
More particularly, the present invention provides a ventilation system for a metal-air battery having a preferred output current density with the fan on of about 50 to 200 ma per square inch of air cathode surface. Each opening preferably has a length to width ratio where the length is greater than about twice the width, with each opening having a length of about 0.3 to 1.5 inches and a width of about 0.03 to 0.3 inches. The openings are preferably sized in the aggregate to permit a flow rate therethrough of about 20 to 80 cubic inches per minute when forced by fan having a capacity of about 100 to 3000 cubic inches per minute.
When the fan is turned off, the openings are sized to slow the rate of diffusion therethrough such that the drain current density is less than 1 ma per square inch of air cathode surface. The preferred ratio of the output current density to the drain current density of the battery is at least 100 to 1. The flow rate with the fan off is preferably about 0.01 to 0.2 cubic inches per minute or less.
According to another aspect of the invention, a metal-air battery is provided that includes a recirculating air distribution system within a metal-air battery housing that minimizes the length of the air distribution path to the air electrode surfaces and minimizes the variation of the concentration of oxygen in the distributed air for all cells, by providing a fan within the battery housing positioned to distribute air to two separate sets of metal-air cells at the same time. In this configuration, all cells of both sets of cells receive air quickly, and the air received is of more uniform oxygen concentration because the air paths are shorter than in previous configurations utilizing the same number of cells.
In the housing of a battery embodying this aspect of the invention, a fan defines a flow axis from a negative pressure side of the fan to a positive pressure side of the fan. The battery further includes at least one ventilation opening in the housing; a plurality of metal-air cells within the housing, at least a first cell being located on a first side of the fan flow axis and at least a second cell being located on a second side of the fan flow axis; a first air path extending from the positive pressure side of the fan along an air electrode side of the first cell and to the negative side of the fan; and a second air path extending from the positive pressure side of the fan along an air electrode side of the second cell and to the negative side of the fan; the fan supplying air to both the first and second air paths at the same time.
The ventilation opening or openings utilized in this embodiment can be of the type described for the first embodiment, or of the type described in U.S. Pat. No. 5,356,729, or can be of the type utilizing an air door. Preferably, two elongate passageways are ported to each side of the fan, and have a length and diameter selected to substantially eliminate diffusion therethrough when the fan is turned off.
In another aspect of the invention, a metal-air cell is provided that includes a micromachine air mover. The micromachine air mover is a pump created from layers of etched silicon to form a three-dimensional structure with sealed cavities. The pump includes a deformable diaphragm that vibrates to generate airflow into a pump chamber. By actuating the diaphram, the pump chamber changes volume and air flow is generated across the metal-air cells.
Thus, it an object of the present invention to provide an improved ventilation system for a metal-air cell or battery.
It is a further object of the present invention to provide an improved method for supplying reactant air to a metal-air cell or battery.
It is a further object of the present invention to provide an air manager apparatus and method that maintains a more stable water vapor equilibrium across the air cathode of a metal-air cell while still providing new oxygen needed for operation of the cell at desired power levels.
It is a further object of the present invention to provide an air manager system that does not require a mechanical air door.
It is a still further object of the present invention to provide an air vent for a metal-air battery housing that substantially eliminates diffusion therein when the fan is turned off.
It is a further object of the present invention to provide a recirculating air distribution system within a metal-air battery housing that delivers oxygen-rich air to all cells in an efficient manner.
Other objects, features and advantages of the present invention will become apparent upon reviewing the following description of preferred embodiments of the invention, when taken in conjunction with the drawings and the appended claims.